A typical circuit board assembly process involves creating a circuit board and populating the circuit board with a variety of circuit board modules. In general, a circuit board manufacturer combines layers of non-conductive material (e.g., fiberglass or plastic) and conductive material (e.g., copper) into a rigid sheet. The manufacturer then mounts modules, i.e., circuit board components such as integrated circuits (ICs), capacitors, resistors, connectors, etc., onto installation locations (part attachment fields) of the circuit board in order to form a circuit board assembly. Finally, the manufacturer tests (and perhaps programs) the circuit board assembly to verify proper operation prior to releasing the circuit board assembly for commercial use.
On occasion, a manufacturer may determine that a particular portion of a circuit board assembly (e.g., a particular module installation location, an etch or trace location, a component, etc.) is a cause of improper circuit board assembly operation. In some situations, the cause is excess solder or debris on the surface of the circuit board (e.g., solder flux remnants and impurities). For example, a particular mounting process that mounts a lead frame IC package to a particular installation location of a circuit board can use a particular solder that leaves a large amount of solder flux residue on the surface of the circuit board. Such residue can lower the surface insulation resistance (SIR) of the circuit board causing a large amount of leakage current to flow between solder joints of the circuit board assembly. Often, the manufacturer can visually detect the residue and remove it (e.g., by washing the circuit board with a special cleaning solution or bath, by blowing air under and around the lead frame package, etc.). Furthermore, the manufacturer can visually inspect other circuit board assemblies to determine whether such residue is common to the circuit board assembly process or an isolated incident. If the residue is common to the circuit board assembly process, the manufacturer can modify the process to remedy the situation. For example, the manufacturer can reduce the residue by improving a cleaning procedure performed on the circuit board assemblies after the mounting process, or by changing the type of solder or flux used during the mounting process.
Some circuit board manufacturers perform stress tests on a sample of circuit board assemblies to determine how the circuit board assemblies will behave after an extended period of real-world use. That is, the manufacturers put the sample of the circuit board assemblies through a variety of environmental stresses (e.g., high temperatures, high humidity, high or prolonged vibration, etc.) which attempt to duplicate extreme real-world operating conditions, and/or accelerate aging of the circuit board assemblies. The manufacturer interprets the results of such tests as an indicator of the suitability of the circuit board assembly process and hopes that such tests will reveal circuit board assembly anomalies that could surface after a period of use. For example, the manufacturer may determine that a circuit board assembly process is unsuitable because solder joints formed during the module mounting process tend to weaken and break over time. The manufacturer can then improve the circuit board assembly process in order to avoid such anomalies in the future (e.g., by changing the temperature applied during the soldering process, by changing the type of solder used, by changing the dimensions or tolerances of pins and/or vias forming the solder joints, etc.).
Some circuit board manufacturers form test boards for testing particular attributes of a circuit board assembly process. One such test involves validating continuity between a test board and a special module which is mounted to the test board using a new circuit board assembly process. In this test, a manufacturer creates a test board, and solders the special module to a designated test board location in order to form a single conductive pathway through all of the solder joints holding the module to the test board. That is, conductive material within the module and within the test board connects the solder joints to form a single daisy chained pathway. After completion of the assembly process, the manufacturer verifies that the module and the test board properly form the single low-resistance pathway (i.e., verify continuity from one end of the pathway to the other), and subsequently subject the test board to a series of stresses that age the test board. Then, the manufacturer tests continuity between the module and the test board (i.e., repeat the continuity test) in order to determine whether the assembly process adequately mounted the module to the test board (i.e., in order to confirm that the assembly process mounts the module to the test board sufficiently to withstand the stresses).
Another test involves a manufacturer forming a comb structure on a test board using a particular circuit board assembly process. The comb structure generally mimics line width and spacing patterns to be implemented on particular circuit boards for commercial use. Such a comb structure includes conductive material on a surface of the test board. The conductive material forms an open pattern of straight, laterally-extending etch runs which can receive a first voltage (e.g., VSS of 10 to 100 volts) and a second voltage (e.g., VGND of 0 volts) in an interleaved manner. A manufacturer then applies the first and second voltages to the comb structure, and measures leakage current between the etch runs. In an optimal situation, the resistance is expected to be high, e.g., on the order of 106 to 108 ohms per square centimeter. In this configuration, cells formed between a first voltage etch and a second voltage etch provide straight voltage gradient lines which extend longitudinally between the laterally-extending etch runs (i.e., perpendicularly to the laterally-extending etch runs). The manufacturer then determines the surface insulation resistance (SIR) for the area of the comb structure by dividing the voltage difference (e.g., 10 volts) by the measured leakage current. This SIR is an indicator of the circuit board which the manufacturer can routinely expect when manufacturing commercial circuit boards having similar line width and spacing patterns. Typically, circuit board assembly processes that provide low SIRs are more susceptible to signal integrity anomalies, i.e., damage to the circuit boards from current leakage over a long-term can tend to weaken insulation properties between particular etches or traces resulting in circuit board assembly failures after a period of normal operation.
Occasionally, if there is surface area available, a circuit board manufacturer may place such a comb structure in a coupon on a commercial circuit board away from any mounted parts. This enables the circuit board manufacturer to obtain SIR information on the actual commercial circuit board. The Institution for Interconnecting and Packaging Electronic Circuits (IPC) of Northbrook, Ill. and other electronic trade organizations provide other test patterns and procedures for assessing the quality of circuit board assembly processes.
A circuit board may properly pass initial tests and inspections prior to its release by a circuit board manufacturer only to fail after a period of time (e.g., a year) of normal operation. The circuit board manufacturer may be able to isolate the cause of the failure to a particular module or module installation location. Occasionally, the circuit board manufacturer may be able to identify the exact cause of the failure. For example, if the module has a lead frame package, the manufacturer may be able to visually inspect the solder joints connecting the module to the circuit board and determine whether any debris or contaminants have caused a large amount of leakage current to flow between two solder joints. If such is the case, the manufacturer may be able to modify the circuit board assembly process (e.g., more thoroughly clean or wash the circuit boards prior to testing and shipping).
However, in some situations, the manufacturer may be unable to visually inspect a module or module installation location. For example, Ball Grid Array modules tend to have several rows and columns of solder joints and when mounted, reside only 0.005 to 0.020 of an inch over the circuit board, thus preventing visual inspection of each solder joint. Although the manufacturer may be able to inspect the module and module installation location using other means (e.g., using X-ray technology), such other means are often time consuming, expensive and less reliable than visual inspection. Accordingly, it may be prudent for the manufacturer to take additional testing steps prior to manufacturing a newly designed circuit board on a large scale, particularly if the circuit board includes a new or questionable technology such as (i) implementing a finer contact pitch (e.g., a contact pitch of 1.0 mm, 0.8 mm or 0.6 mm on a large BGA module), or (ii) using new solder/flux material.
Unfortunately, the conventional approaches to forming a test board in order to test particular circuit board assembly processes suffer from certain drawbacks. For example, a circuit board manufacturer which forms a daisy chain configuration between a special module and a test board can test the configuration to verify proper installation of the module, i.e., that continuity exists through a single conductive pathway formed by daisy chaining solder joints between the module and the test board). However, the manufacturer typically finds the configuration to be of little use in determining the leakage current or SIR of the test board since a relatively large amount of current generally passes through the pathway when the module is properly installed on the test board.
Furthermore, a manufacturer which forms a comb structure on a test board (i.e., open, interleaved, straight etch runs of positive and ground voltages) can measure leakage current through the comb structure, but the comb structure is not a normal circuit feature. That is, the comb structure does not accurately duplicate certain configurations or structures occurring on operating circuit boards with operating components, coatings, etc. For example, the comb structure is not well suited to the smaller dimensions used with current and future Ball Grid Array (BGA) technologies which provide multiple module/circuit board connections in a relatively small area. Rather, the comb structure merely mimics general line width and spacing patterns. Accordingly, leakage current measured through the comb structure, and any resulting SIR calculated from that leakage current are not accurate representations of electrical properties existing where a BGA module is mounted to the circuit board, i.e., a location that is susceptible to SIR anomalies (e.g., shorts between solder joints due to debris, the closeness of the solder joints, contaminant entrapment, etc.).
In contrast, the present invention is directed to techniques for obtaining an electrical characteristic of a circuit board assembly process which involves mounting a module to an installation location of a circuit board using the circuit board assembly process in order to form a test structure, and measuring leakage current of the test structure in response to an electrical signal (e.g., a voltage of 10 to 20 volts) applied to the installation location. The module mounted on the test structure more closely duplicates the situation of a mounted operating module on an operating circuit board using the same or similar circuit board assembly process than a conventional comb structure on a test board. Furthermore, one can determine the SIR of the test structure based on the electrical signal and the leakage current in order to assess the suitability of the circuit board assembly process. Such a determination cannot be made in the conventional test board approaches such as that of mounting a module on a test board to form a daisy chain pathway. Accordingly, a circuit board manufacturer can duplicate the physical construction to be employed in an operating circuit board (e.g., a real commercial circuit board device), and allow more accurate measurements of the electrical properties of that circuit board. As a result, the manufacturer can validate a particular circuit board assembly process or determine whether the particular circuit board assembly process is suitable, e.g., certify that the materials used are resistive enough, confirm that the process is xe2x80x9ccleanxe2x80x9d (i.e., not hindered by flux debris or contaminants), test the interaction of materials (e.g., corrosion), test for thermal coefficient of expansion mismatches, etc.
One arrangement of the invention is directed to a system for obtaining an electrical characteristic of a circuit board assembly process. The system includes a test structure that includes (i) a circuit board having a module installation location and an interface location in electrical communication with the module installation location, and (ii) a module which is mounted to the circuit board at the module installation location using the circuit board assembly process. The system further includes a signal generator that couples to the interface location of the circuit board, and that applies an electrical signal to the module installation location of the circuit board through the interface location. Additionally, the system includes a detector that that couples to the interface location, and that measures leakage current of the test structure in response to the electrical signal. The leakage current is itself an obtained electrical characteristic of the circuit board assembly process. Nevertheless, the leakage current and the electrical signal enable determination of surface insulation resistance of the test structure, as another electrical characteristic of the circuit board assembly process.
In one arrangement, the module includes a Ball Grid Array (BGA) package. For this arrangement, the circuit board assembly process includes a BGA component mounting process. In this arrangement, determination of the SIR provides an indication of the adequacy of the BGA component mounting process. In some situations, a circuit board manufacturer can use the techniques of the invention to determine the suitability of a particular BGA component mounting process for a particular pitch or connection density between a BGA module and a circuit board. For example, an extremely low SIR for the test structure can indicate that a module having a particular BGA footprint is unsuitable due to the low SIR.
In one arrangement, the module installation location of the circuit board includes (i) a set of first circuit board contacts which are electrically connected together, and (ii) a set of second circuit board contacts which are electrically connected together. In this arrangement, the module includes (i) a set of first module contacts which connects with the set of first circuit board contacts, and (ii) a set of second module contacts which connects with the set of second circuit board contacts and which is electrically isolated from the set of first module contacts. Preferably, the electrical signal is a voltage (e.g., 10 to 20 volts) applied between the set of first circuit board contacts and the set of second circuit board contacts in order to determine, as the electrical characteristic, a surface insulation resistance of the test structure based on the voltage and the leakage current.
In one arrangement, the module and the circuit board form a set of repetitive cells when the electrical signal is applied to the module installation location of the circuit board. Each cell has voltage gradient lines which radially extend between solder joints connecting the module to the circuit board. Since the voltage gradient lines of the cells extend radially, the cells are different than conventional cells having laterally extending voltage gradient lines. Such repetitive cell information enables improved predictability for similar implementations. In one arrangement, there are a large number of cells (e.g., several hundred) which provide an amplified effect for easier measurement and quantification, i.e., that provide values that are high enough to be measured by standard test equipment.
In one arrangement, the module installation location of the circuit board includes an array of contacts and a set of electrical connections. In this arrangement, each contact of an inner portion of the array has four immediately adjacent neighboring contacts. Additionally, the set of electrical connections connects each contact of the inner portion to a contact other than one of the four immediately adjacent neighboring contacts of that contact.
One arrangement involves exposing the test structure to an extreme environmental condition (e.g., high temperature and high humidity) for a period of time in order to stress the test structure prior to applying the electrical signal and measuring for leakage current. Power can also be applied to the test structure (e.g., a voltage bias) as an accelerating factor during such exposure. Such exposure provides for accelerated aging or worst case operating conditions for the test structure in order to determine how the test structure or a similar circuit board assembly will perform under extreme operating conditions.
The features of the invention, as described above, may be employed in circuit boards, systems and related devices such as those manufactured by EMC Corporation of Hopkinton, Mass.